1. Field Of The Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of dichlorobenzene (DCB) isomers. More specifically, the invention relates to a process for extracting the meta isomer of dichlorobenzene (m-DCB) from mixtures of DCB isomers employing as the adsorbent X zeolites containing mixed alkali metal cations.
2. Background Information
The individual isomers of dichlorobenzene are useful in a variety of reactions, e.g., as intermediates for making pesticides, pharmaceuticals, peroxides, dyes, etc. The dichlorobenzene isomers are normally prepared by the non-catalytic nuclear chlorination of benzene or chlorobenzene in various solvents. The chlorinating agent comprises certain Lewis acid halides, e.g., hydrogen chloride in both liquid and vapor phase systems. The direct chlorination of chlorobenzene or benzene produces a mixture of dichlorobenzene isomers.
Separation of m-DCB from the other isomers of dichlorobenzene by conventional distillation techniques is difficult due to the close boiling point range of these isomers. The following table shows the boiling points of the isomers.
TABLE 1 ______________________________________ DCB Isomer Boiling Point (.degree.C.) ______________________________________ o-DCB 180.4 m-DCB 173.0 p-DCB 174.1 ______________________________________
While it is possible to separate o-DCB by distillation from the isomer mixture produced by the process of direct chlorination of chlorobenzene or benzene referred to above, the desired product m-DCB cannot be easily separated from p-DCB by distillation.
Fleck et al U.S. Pat. No. 2,958,708 discloses the adsorptive separation of ortho-dichlorobenzene from meta-dichlorobenzene with sodium-exchanged X zeolite adsorbent, which can be partially exchanged by calcium, magnesium, strontium or barium ions, and an aromatic hydrocarbon, preferably chlorobenzene, desorbent. The process is m-DCB rejective, i.e., m-DCB is non-adsorbed. Another meta-DCB rejective process using at least partially lithium-exchanged sodium-X zeolite and either toluene or 3,4-dichlorotoluene desorbent is disclosed in Japanese Public Disclosure 268636/86. It is also disclosed therein (Table 1 Adsorbent No. 1), that meta-DCB is preferentially adsorbed over para-DCB, when sodium-exchanged X zeolite is the adsorbent with toluene as desorbent, but the selectivity of meta-DCB over ortho-DCB is so low that separation of meta-DCB from isomer mixtures containing ortho-DCB would not be effective. It is apparent that meta-DCB could not be removed in pure form without first removing the ortho-DCB by distillation.
Japanese Public Disclosure 150524/83 discloses the rejective separation of meta-dichlorobenzene from its isomers with a sodium, potassium- or silver-potassium-exchanged Y type zeolite containing at least 90% potassium ions and either 3,4-dichlorotoluene or 4-chloroorthoxylene as desorbent.
Another meta-rejective absorptive (sic) separation is disclosed in EP 278,680 utilizing a high-silica zeolite, TPZ-3 in either sodium- or acid-form, and conventional desorption procedures.
U.S. Pat. No. 4,571,441 discloses a meta-rejective preparation using a Y zeolite exchanged with silver and/or copper ions.
Japanese Public Disclosure 5155/62, to the extent that it appears pertinent, discloses the use of chlorobenzene as desorbent in an adsorptive separation of halogenated aromatic isomers with an X-type zeolite.
Methods for forming the crystalline powders into agglomerates are also known and include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to a high purity zeolite powder in wet mixture. The blended clay-zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. Clays of the kaolin type, water permeable organic polymers and silica may also be used as binders.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
Of the above references, only Japanese Public Disclosure 268636/86 discloses a meta-dichlorobenzene extractive process. However, selectivity over the ortho isomer with NaX zeolite is so low (.alpha..sub.o/m =0.94--Table 1) that meta-DCB could not be effectively separated from ortho-DCB. We have discovered that improved selectivity for m-DCB and particularly over ortho-DCB, can be achieved with an adsorbent which contains certain mixtures of sodium and potassium ions at the ion exchangeable sites. It was entirely unexpected that m-DCB selectivity could be reversed from that of K-exchanged zeolites and also that relativity between m-DCB and o-DCB could be achieved.
A meta-selective process is preferred for purification of DCB isomer mixtures because high purity m-DCB can be obtained when it is extracted over the other isomers.